Please forgive me, I am an electrician who started off on a bench,...electronics.

I am wrapping my little tiny brain around something I don't comprehend. We have this motor controller at work with failing chokes on them and I don't have the means to copy and paste or draw a schematic.

We're having this trouble I have seen before, but not to this extent.

I am trying to relate inductance vs. resistance. This coil is supposed to read 1.8uH/Ls and 7.4Ω Rs. We've seen high failure rates where the coils read low on Ls, and fine is Rs. The faults I find are find mostly low in Rs and spot on in Ls. I found a coil today reading 314.07uH in Ls, and 6.08 Rs. I have NEVER seen readings like that in Ls/Rs relationship. I'm using an HP LSR meter which is spot on.

I have never run into chokes causing this type of trouble. Just taking into equation this one coil in particular,....how can it be 314uH, and Rs of 6.08??????

It's not damaged in any way. Most of the time the GenRad will catch it, sometimes they get through to functional test where the motor won't even bump let alone start. I will try to post a partial schematic tomorrow.

Not posting a schematic and throwing out such a general question, I feel ridiculous. I apologize.

Anyone care to take a stab?

 

Sounds to me as if you've received a bad run of inductors, do you have any new ones you can go through and compare quite a few?

 

How about a shorted turn(s)? If the tester measures dI/dt to infer the inductance, this might give the excessive uH. It would also lower the resistance by the shorted turn(s) not contributing to the resistance reading.

 

Nothing to do with the problem, but have you calibrated your LCR meter?

What kind of power is going through these inductors when in use? It was stated they are in a motor application?

 

Shorted turns on inductor winding usually reduce inductance.

Never seen one with increased inductance.

 

Sounds like the wrong ferrite powder mix. It is usually a mix of low carbon iron, epoxy binders that activate with heat and pressure, and small amounts of other magnetic and non magnetic minerals and metals to adjust the inductance range.

I worked as QC for a short time in a plant making inductor cores and transformer E cores.

Look to the manufacturer to have screwed something up. They are supposed to sample lots, before and after heat treatment. The wrong numbers before a heat treatment means they should be rejected and ground up and sent back as a 'used' powder mix, and reused in other batches as an additive powder.

 

Shorted turns on inductor winding usually reduce inductance.

Never seen one with increased inductance.

Never said it HAD increased inductance. Do you believe everything a meter tells you?

 

Sounds like the wrong ferrite powder mix.

Can I get some of this 'wrong' ferrite powder mix that gives 200 times the inductance. I'm sure I could find a market for some of that!

 

I did a quick simulation to see what happens when an inductor has developed short turns.

It confirms what DonQ has mentioned about the effect of di/dt. The inductor current, instead of rising uniformly, jumps immediately to a certain value and then ramps up gradually.

This rate of change of the current does become much less than that of a normal inductor with no shorted turns. The meter will see this dI/dt as an inductor with higher inductance if it ignores the initial current jump and relies solely on using the remaining dI/dt slope to calculate the inductance.

 

@76nemo,

This coil is supposed to read 1.8uH/Ls and 7.4Ω Rs.

Something is not match here. A common 1.8uH inductor should be wound with heavy gauge wire and with at most a few turns. There is no reason why it would have a resistance value as high as 7.4Ω.

I found a coil today reading 314.07uH in Ls, and 6.08 Rs.

It is apparent that your HP meter might not give you a reliable reading on faulty inductor with shorted turns.

There is however, another type of instrument that measures the physical response of a healthy inductor in response to external stimulus, i.e. ringings.

This is one I made and it is very sensitive in detecting short turn. On an healthy inductor with 100 turns, it will usually detect six or seven rings but with just one shorted turn, the detectable ringing reduces to one.

It can be use on all sort of transformers and coils to see if there is any shorted turns which is normally not detectable using ohmic measurement.

See here:

LOPT / FBT Testers

with full schematic for anyone interested to build one.

Edited: I might have spoken too soon for the OP. His inductor in question is only 1.8uH. I just tested my tester on a 18uH inductor and its LEDs shows no indication of ringing.

I got two LEDs(rings) with an inductor of 47uH so 47uH is probably the lowest limit of inductance that the circuit can detect ringings on inductors.